The Triplet Code (AQA A Level Biology): Revision Note
Exam code: 7402
The triplet code
The DNA nucleotide base code found within a gene is a three-letter, or triplet, code
Each triplet codes for one amino acid
There are four bases (A, G, C, T), so there are 64 different triplets possible (43), yet there are only 20 amino acids that commonly occur in biological proteins
This results in multiple triplets coding for the same amino acids; thus, the code is said to be degenerate (this can limit the effect of mutations)
Some triplets code for start (TAC – methionine) and stop signals
These signals tell the cell where individual genes start and stop
This ensures the cell
reads the DNA correctly as the code is non-overlapping
can produce the correct sequences of amino acids (and therefore the correct protein molecules) that it requires to function properly
The genetic code is universal, meaning that almost every organism uses the same code (there are a few rare and minor exceptions)
This means that the same triplet codes for the same amino acids are found in all living organisms

Examiner Tips and Tricks
Be careful to determine whether you have been given DNA triplets or mRNA codons to examine in a question. Remembering that the mRNA codon will have been transcribed from the DNA triplet, and so it will have a complementary base sequence to the original DNA sequence.
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